//
// Created by Dell on 2021/12/11.
//

//#include <stdio.h>
#include <math.h>
#include <iostream>
#include <vector>
#include <stack>
#include <algorithm>
#include <string.h>

using namespace std;


int n,m,k,temk;
int ans = 0;
string re,sh,ss;
int main()
{
    cin>>k>>re;
    getchar();
    getline(cin,sh);
    ss = sh;
    m = re.length();
    for(int i = 0;i<ss.length()-m+1;++i){
        if(ss.substr(i,m) == re){
            ++temk;
            if(temk == k){
                for(int j = i;j<=i+m-1;++j){
                    int mod;
                    if(ss[j] >='a'&&ss[j]<='z'){
                        mod = 'z' - 'a' + 1;
                        ss[j] = char(((ss[j]-'a')+1)%mod + 'a');
                    }else{
                        mod = 'Z' - 'A' + 1;

                        ss[j] = char(((ss[j]-'A')+1)%mod + 'A');
                    }
                }
            }
        }
    }
    if(temk >= k){
        cout<<ss;
    }else{
        cout<<"F";
    }
    return 0;
}
//2 re
//Thre is re
